In a magnetic disk drive, a read element, such as an inductive head or, more recently, a magneto-resistive ("MR") head, is used to detect magnetic flux changes on the disk, representative of recorded digital data. The resistance of the MR element inversely proportional to the strength of the magnetic flux such that the resistance increases as a flux transition on moving media approaches the element. (The resistance of the element is also proportional to its temperature.) When a constant current is passed through the MR element, the detected voltage across the element represents the analog read signal with its alternating-polarity pulses.
It is the role of the read channel to separate the raw analog read signal from extraneous noise and then convert the resulting signal into digital bits which can be converted into useful data. One source of noise is known as a thermal asperity ("TA") which occurs when an MR element physically strikes a blemish or asperity on the surface of the disk. The result is an immediate and significant increase in the temperature of the element with a corresponding increase in its resistivity. This, in turn, results in a voltage transient in the analog read signal which can mask valid flux changes as the transient decays exponentially and can introduce errors into the read signal. For accurate reproduction of the data, the errors must be detected and corrected by an error correcting code ("ECC"). FIG. 1A is a plot of an analog read signal 100 showing a TA transient 110. When the MR element strikes an asperity, the DC offset of the read signal increases to a point of saturation 112 then decays 114 back to a level within the operating range 102 of the DC offset. During the decay period 116, which can last about 100 to 1000 channel bits, the detected digital data will contain errors.
Although ECC circuitry may be used to detect and correct "hard" errors caused by thermal asperities (as well as to detect and correct "soft" errors caused by other sources of channel noise), doing so increases the cost and complexity of the ECC circuitry and increases the amount of ECC data added to the user data recorded onto the disk, thereby reducing the usable storage capacity of the disk. Consequently, efforts have been made to devise dedicated TA detection and compensation methods. For example, in one common method of detecting a TA (as described in commonly-assigned U.S. patent application Ser. No. 08/576,742 to Armstrong et al. entitled ON-THE-FLY ERROR CORRECTION USING THERMAL ASPERITY ERASURE POINTERS FROM A SAMPLED AMPLITUDE READ CHANNEL IN A MAGNETIC DISK DRIVE, hereinafter referred to as "Armstrong" and hereby incorporated by reference), a TA is assumed to have occurred when analog-to-digital converter ("ADC") samples in the read channel saturate. However, when ADC saturation is relied upon for TA detection, some "mild" TA events may go undetected and the data they mask remain uncorrected. For example, detection of a TA through saturation of a 6-bit ADC may occur at 31 least significant bits ("lsb") of the ADC sample. However, if it is desirable to detect a small TA of as little as 20 least significant bits, not only can saturation detection not be sufficiently sensitive to true TAs, but it can be overly sensitive to false TAs. In fact, there may typically be only about 3.5 standard deviations of noise between the signal peak and the saturation level. Therefore, the probability is very high that an ADC sample will saturate due to noise: perhaps an unacceptable one sample out of every 1000 to 10,000 will saturate and register as a TA. And, while reducing the saturation level might allow smaller TAs to be detected, the number of false, noise-caused TA detections will increase still further by the reduced headroom between the analog signal peak and the saturation level.
Regardless of the detection method employed, once a TA has been detected, an effort must be made to recover data which is masked or distorted while the TA decays. Examples of compensation (again, as described in Armstrong et al.) include: increasing the pole of an AC coupling capacitor; holding parameters of the timing and gain control loops in a constant state; and/or increasing the headroom of the ADC. FIG. 1B illustrates the effect of TA compensation on the analog read signal 100. Elevating the pole of the AC coupling capacitor reduces the apparent TA time constant, thereb reducing the amount of time the ADC is saturated 122. The DC offset 124 thus decays more rapidly to the proper operating level 102, reducing the number of data bits corrupted by the TA 120. Ideally, the corrupted data bits can be detected and corrected by ECC circuitry. Additionally, after the ADC samples come out of saturation, the ADC headroom is increased to scale the samples to fit within the acceptable operating range of the read channel, thereby reducing the load on expensive and complex ECC circuitry.
As with the known TA detection methods, the foregoing compensation techniques also have drawbacks. Increasing the ADC headroom by decreasing signal gain tends to increase the number of soft errors due to reduced ADC resolution. Moreover, the number of soft errors may exceed the error correction capability of the ECC circuitry and force the read channel to attempt to re-read the data on subsequent revolutions of the disk. As will be appreciated, any resulting increase in access time is generally undesirable. And, additional soft errors can also occur through transients introduced into the signal when the ADC headroom is readjusted back to its normal operating range.